Field of the Invention
The present invention relates to a developing apparatus, a developing method, an image forming apparatus, and an image forming method.
Description of the Related Art
Higher quality of electrophotographic images has been required, and suppression of generation of “fogging” caused by toners adhering to non-image portions of electrophotographic images has been required.
A reduction in size of image forming apparatuses has recently been required. One of effective measures to satisfy this requirement is a reduction in size of a developing apparatus, which occupies a considerable volume of an image forming apparatus.
One specific method of achieving a reduction in size of the developing apparatus is use of a system without a cleaner for an electrophotographic photosensitive member (hereinafter also referred to as a “cleaner-less system”). In such a system without the cleaner for an electrophotographic photosensitive member, however, a toner adhering onto non-developed portions, e.g., non-electrostatic latent image portions of the electrophotographic photosensitive member and remain on the electrophotographic photosensitive member even after a transferring step may adhere to a charging member and contaminate the charging member. Accordingly, adhesion of the toner to non-developed portions of the electrophotographic photosensitive member should be suppressed more significantly to attain formation of electrophotographic images with higher quality in the cleaner-less system.
Japanese Patent Application Laid-Open No. 2002-23485 discloses techniques for providing good images by way of a developing roller including a second resin coating layer of 1010 Ω·cm or more not containing an electro-conductive fine particle on a first resin coating layer of 106 Ω·cm or less.
The present inventors, who have conducted extensive research, have found that when the developing roller described in Japanese Patent Application Laid-Open No. 2002-23485 is used, adhesion of a toner to non-developed portions of the electrophotographic photosensitive member may be effectively suppressed. This is probably because the developing roller described in Japanese Patent Application Laid-Open No. 2002-23485 has a layer having high electric resistance on the surface thereof. Due to the high electric resistance of the developing roller, charges applied to the toner present between an electrophotographic photosensitive member and the developing roller barely leak to the side of the developing roller, so that the toner can keep high charge.
Unfortunately, the present inventors have conducted further research and found that the developing roller may insufficiently control the amount of the toner to be conveyed by the developing roller with a toner layer thickness controlling member under low humidity environments in which the toner readily undergoes friction charge (hereinafter this phenomenon is referred to insufficient control). Namely, the insufficient control is probably caused for the following reasons 1) and/or 2):
1) The electric resistance of the surface of the developing roller provides excessively high charges of the toner carried on the developing roller, and the highly charged toner attracts the toner not carried on the developing roller by an electrical attractive force.2) The excessively charged toner adheres to the surface of the developing roller due to an image force to roughen the surface of the developing roller, so that an unintendedly large amount of the toner is disposed on the developing roller.
Such insufficient control may cause defects called ghost and spotted or wavy unevenness of non-image portions in electrophotographic images.